The most straightforward thing (if I were trying to attempt the same project) that I would do would be to create a post for each picture. Then it would be a matter of using e.g. the WordPress media/image uploader, inserting it into the post, giving it a title and description and publishing the post.

Then I’d install something like GamerZ’s WP-PostRatings which is literally used for rating each post, and let users use this to for for each picture. However, it wouldn’t be quite the ‘keep-and-toss’.